Combustion catalysts have long been known and palladium-containing catalysts have been identified as among the most active catalysts for combustion. The compound Pr4PdO7, described by Chou et al. in U.S. Pat. No. 5,102,639, is one example of a Pd-containing catalyst. Dalla Betta et al. in U.S. Pat. No. 5,259,754 disclosed a Pd-on-zirconia catalyst; the zirconia could be a washcoat applied on a metal substrate. Euzen et al. in U.S. Pat. No. 6,284,210 described combustion catalysts containing: a metallic or ceramic monolithic substrate, an oxide coating (typically alumina), iron and cerium deposited on the oxide, and Pd and Pt.
Hanakata et al. in U.S. Pat. No. 5,577,906 disclosed a combustion catalyst in which Pd and/or palladium oxide are the main components of the active catalyst. These workers suggested a long list of possible cocatalysts including platinum, cerium and terbium. The active catalyst components can be mixed with a heat-resistant substance and deposited on a durable ceramic or metal support.
Not all combustion catalysts are based on noble metals. Other well-known catalysts are mixed oxides of perovskite structure and hexa-aluminates. General background information about catalytic materials for combustion can be found in “Catalytic Materials for High-Temperature Combustion”, by M. Zwinkels et al., in Catalysis Review—Science and Engineering, 35(3), pp. 319-358, 1993. In a recent publication, Zarur et al. disclosed a new, highly active combustion catalyst made by sol-gel processing in reverse microemulsions. See “Reverse microemulsion synthesis of nanostructured complex oxides for catalytic combustion,” Nature, 403, 55-57 (2000). The resulting ceria-promoted, barium hexa-aluminate nanoparticles were tested in the combustion of a reactant stream of 1% methane in air at a gas hourly space velocity (“GHSV”) of 60,000 h−1. In this system, full methane conversion was observed at 600° C.
The aforementioned patents discuss methods of preparing new catalysts. Some workers, however, have developed methods of refurbishing used catalyst structures. For example, Zahn et al. in U.S. Pat. No. 5,877,107 disclosed a process of cleaning or removing a catalyst coating from a catalytic converter by the use of ultrasound and flushing a honeycomb body with a chemically or mechanically active fluid.
While the invention described below focuses primarily on combustion catalysis, a few catalyst systems outside this field may be mentioned here. Twigg, in U.S. Pat. No. 4,581,157 described steam reforming catalysts that can have a metal substrate coated with a secondary oxide support that may contain a grain growth inhibitor such as cerium or praseodymium, and a group VIII metal active catalyst is disposed over the oxide. Meguerian et al. in U.S. Pat. Nos. 4,021,372 and 4,104,360 disclosed a process of making a catalyst for reducing nitrogen oxides in exhaust gas. In their process, a metal is plated on a metal support, heated in an inert atmosphere to interdiffuse the metals and then oxidized to form a surface oxide containing Cu, Ni, Fe, or Cr. Pfefferle in U.S. Pat. No. 5,466,651 disclosed a catalyst made by sputtering an admixture of a precious metal and a base metal oxide on a metal mesh support.
Barnes et al. in U.S. Pat. No. 6,488,907 disclosed catalytic partial oxidation methods and partial oxidation catalysts having a diffusion barrier coating on selected metal supports. The partial oxidation catalyst contains an alloy support that is heated in air or oxygen and then coated with a catalyst metal such as Rh, Pt, Ru, Ir, and Re. Barnes et al. suggested the use of supports in the form of gauzes, honeycombs or other configurations having longitudinal channels permitting high space velocities and a minimal pressure drop.
Barnes et al. reported examples of partially oxidizing methane at a methane:O2 ratio of about 2 at gas hourly space velocities (GHSVs) of 100,000-140,000 hr−1 to produce CO and H2. It is well-known from the work of Barnes et al. and others that high gas space velocities can be used to obtain good yields in partial oxidation reactions even with catalysts with low surface area supports; however, it is also commonly believed that similarly high gas velocities cannot be successfully employed in comparable systems to obtain good yields in combustion reactions. Thus, relatively slower GHSVs and high surface area supports are the universal choice for catalytic combustors.
Despite these and many other efforts over the years, there remains a need for new catalysts, catalyst supports, and methods of combustion that reduce the cost and/or improve performance of combustion systems.